(a) Technical Field
The present invention discloses 2-arylbenzothiophene derivatives or pharmaceutically acceptable salts thereof, a preparation method thereof, and a pharmaceutical composition for the diagnosis or treatment of degenerative brain disease, containing the same as an active ingredient.
(b) Background Art
With an increase of the aged population and an increase in the life span of human beings, the incidence of Alzheimer's disease (hereinafter, referred to simply as “AD”) has been increasing. AD is one of representative degenerative neurological diseases and Dr. Alois Alzheimer first noticed characteristic signs and symptoms of AD in 1906. AD is known as one of the most common causes of dementia among older people, causing memory loss and other cognitive deficits (MaKhann et al., Neurology, 1984, 34, 939-944). Incidence of AD for prolonged periods may increase risks for complications and lead to even death. AD can strike people as young as 40-50 years of age, and the prevalence of AD increases with age, with estimates of the affected population reaching as high as 40-50% by ages 85-90. (Evans et al., The Journal of the American Medical Association, 1989, 262, 2551-2556; Katzman, Neurology, 1993, 43, 13-20).
Recently, results of research into AD are being reported by many scientists and development of pharmaceutical products for prevention and treatment of various diseases is actively under way. However, no available medication that has been developed so far can completely cure the disease once it onsets but can only slow delay the progression of the disease. Accordingly, the best way to cope with AD is to slow or delay the AD progression through early diagnosis. Since pathological changes of AD probably occur about 7 years before AD symptoms are noticeable, early diagnosis of the AD and the follow-up treatment for the early diagnosis are keenly needed.
Postmortem slices of AD patient's brain tissue exhibit the presence of amyloid plaque and neurofibrillary tangles (NFTs). The amyloid plaques are extracellular amyloid peptide deposits and the NFTs are intracellular deposits of microtubule associated tau protein. Although the issue as to which of the two types of lesions in the brain appears first has remained controversial, the presence of amyloid plaques as an early sign of AD is undoubtedly accepted by the majority of researchers. The presence of β-amyloid may cause considerable biochemical changes, leading to deposits of other proteins, thereby activating phagocytosis of microglial cells, and resulting in death of neurons and incurable loss of cognitive functions.
The initial deposition of β-amyloid probably occurs long before clinical symptoms are noticeable. The currently recommended “minimum microscopic criteria” for the diagnosis of AD is based on the number of neuritic plaques found in brain (Khachaturian, Arch. Neurol., 1985, 42, 1097-1105). Unfortunately, assessment of neuritic plaque counts must be delayed until after death. The amyloid of these neuritic plaques is a peptide composed of amino acids 39-43 called the β-amyloid that is arranged in a predominately beta-pleated sheet configuration (Kirschner et al., Proc. Natl. Acad. Sci., 1986, 83, 503-507). Specifically, β-amyloid 42 demonstrates higher neurotoxicity and much higher formation rate of amyloid plates than β-amyloid 40, which is due to its hydrophobic amino acid residues (Yoshiike, Y.; Takashima, A. Am. Soc. Biochem. Molecular Biol., 2003, 278, 23648-23655). Amino Acid Sequence for β-amyloid is as follows:
TABLE 1A 42A 40 1 2 3 4 5 6 7 8 910AspAlaGluPheArgHisAspSerGlyTyr11121314151617181920GluValHisHisGlnLysLeuValPhePhe21222324252627282930AlaGluAspValGlySerAsnLysGlyAla31323334353637383940IleIleGlyLeuMetValGlyGlyValVal4142IleAla
Up to recently, there have been no antemortem probes for brain β-amyloid deposits of AD patients or patients who are suspected of having AD. Brain amyloid deposits were only analyzed by staining the brain sections after the patients died. Brain amyloid is readily demonstrated by staining brain sections with Thioflavin S of Formula I or Congo red of Formula II (Puchtler et al., J. Histochem. Cytochem., 1962, 10, 355-364). Congo red stained amyloid is characterized by a dichroic appearance, exhibiting a yellow-green polarization color. The dichroic binding is the result of the beta-pleated sheet structure of the amyloid proteins (Glenner, G. N. Eng. J. Med., 1980, 302, 1283-1292).

Thus far, diagnosis of AD has been achieved mostly through clinical criteria evaluation, brain biopsies and post-mortem tissue studies. Research efforts to develop methods for diagnosing Alzheimer's disease in vivo, including immunoassay methods and imaging techniques, have been attempted.
Immunoassay methods have been developed to detect the presence of neurochemical markers in AD patients and to detect an AD related amyloid protein in cerebral spinal fluid (Warner, Anal. Chem. 59: 1203A (1987); World Patent No. 92/17152 by Potter; Glenner et al., U.S. Pat. No. 4,666,829; Majocha et al., J. Nucl. Med., 1992, 33, 2184; Majocha et al., WO 89/06242; and Majocha et al., U.S. Pat. No. 5,231,000). One of major disadvantages with antibody probes is a difficulty in getting these large molecules across the blood-brain barrier (BBB). In order to gain access into the brain, using antibodies for in vivo diagnosis of AD would require marked abnormalities in the blood-brain barrier. There is no convincing functional evidence that abnormalities in the blood-brain barrier reliably exist in AD. Radiolabeled amyloid peptides have also been used to label diffuse, compact and neuritic type plaques in sections of AD brain (Edward et al., WO 93/04194). However, these peptides share all of the disadvantages of antibodies in that peptides do not normally cross the blood-brain barrier in amounts necessary for imaging.
As described above, Congo red and Thioflavin S are dyes used to stain β-amyloid deposits in postmortem brain tissues. The binding affinities and specific selectivity of Congo red and Thioflavin S imply possibilities of detecting the presence and amount of β-amyloid deposits in the amyloid in vivo. However, since these compounds have relatively large molecular weights, it is quite difficult for them to cross the blood brain barrier (BBB). In addition, sulfonate salt and quarternary ammonium salt contained in each compound make Congo red and Thioflavin S more difficult cross the blood brain barrier (BBB). Furthermore, Congo red has a diazo group, which is known to be carcinogenic, and is metabolized in the form of a free amine by intestinal bacteria, which is, however, disadvantageous, in that bioavailability is noticeably lowered in a case where a therapeutic compound is orally administered.
Thioflavin S is commonly used in the post-mortem study of amyloid deposition in AD brain where it has been shown to be one of the most sensitive techniques for demonstrating senile plaques. Vallet et al. Acta Neuropatholi., 1992, 84, 170). Thioflavin T of Formula III has been frequently used as a reagent to study the aggregation of soluble amyloid proteins into beta-sheet fibrils (LeVine Prot. Sci. 1993, 2, 404-410). Quaternary amine derivatives related to Thioflavin T have been proposed as amyloid imaging agents, although no evidence of brain uptake of these agents has been presented.

Recently, neutral derivative groups have been developed from Thioflavin T structure including an uncharged 2-arylbenzothiazole structure, and varying a variety of substituent groups led to findings that the neutral derivative groups strongly bind with β-amyloid deposits and easily cross the blood-brain barrier. In addition, other different analogs having diaryl or conjugated diaryl structures that are characteristic of the 2-arylbenzothiazole structure were developed by many researchers, who have studied detection methods and distribution of amyloid in vivo using compounds labeled with radioisotopes such as F-18, C-11, I-123, I-125 or the like, as of Formulas IV-VII. Details of each of the compounds of Formulas IV-VII, including abbreviated names, binding affinity (Ki) values, discoverers and documents cited (?) are listed below: Compound of Formula IV_[11C]PIB, Ki=2.8 nM, Mathis, C. A., Wang, Y., Holt, D. P., Huang, G. F., Debnath, M. L., Klunk, W. E. Synthesis and evaluation of 11C-labeled 6-substituted 2-arylbenzothiazoles as amyloid imaging agents., J. Med. Chem., 2003, 46, 2740-2754; Compound of Formula V_[18F]FDDNP, Ki=0.12 nM, Shoghi-Jadid, K., Small, G. W., Agdeppa, E. D., Kepe, V., Ercoli, L. M., Siddarth, P., Read, S., Satyamurthy, N., Petric, A., Huang, S. C., Barrio, J. R., Localization of neurofibrillary tangles and beta-amyloid plaques in the brains of living patients with Alzheimer disease., Am. J. Geriatr. Psychiatry, 2002, 10, 24-35; Compound of Formula VI_[11C]SB-13, Ki=1.2 nM, Ono, M., Wilson, A., Nobrega, J., Westaway, D., Verhoeff, P., Zhuang, Z.-P., Kung, M.-P., Kung, H. F., 11C-Labeled Stilbene Derivatives as Ab-aggregate-specific PET Imaging Agents for Alzheimer's Disease, Nucl. Med. Biol., 2003, 30, 565-571; Compound of Formula VII_[123I/125I]IMPY, Ki=15.0 nM, Kung, M. P., Hou, C., Zhuang, Z. P., Zhang, B., Skovronsky, D., Trojanowski, J. Q., Lee, V. M., Kung, H. F., IMPY: an improved thioflavin-T derivative for in vivo labeling of beta-amyloid plaques., Brain Res., 2002, 956, 202-210).

The compounds of Formulas IV, V, VI and VII are provided just as some representative compounds of a variety of compounds researched, which were developed for the diagnosis of the onset and progression of AD. The compounds of Formulas IV, V, VI and VII are radioactive tracers labeled with radioisotopes, allowing detection of β-amyloid deposits in vivo by positron emission tomorgraphy (PET) and single-photon emission computed tomography (SPECT).
In order to effectively detect and diagnose β-amyloid deposits in living patients, the following criteria must be satisfied: a high binding affinity for β-amyloid deposits; high selectivity to biological substance analogs such as neurofibrillary tangle (NFT); use of the minimum dosage concentration without toxicity, or the normally amount necessary for tomography using radioisotopes in crossing the blood-brain barrier to finally reach target β-amyloid deposits. In conclusion, it is important to obtain AD amyloid deposits in brain, which can be definitely distinguished from AD brain from normal brain.
Currently, existing methods for quantifying antemortem probes for brain amyloid deposits have been achieved mostly by diagnosis through doctors' clinical consultation and monitoring the effectiveness of therapies targeted at preventing β-amyloid deposits. However, it has turned out that the existing methods are not so accurate and the therapies associated with β-amyloid deposits are pointless once clinical signs are detected. Therefore, it remains of utmost importance to develop safe and specific methods for diagnosing AD by imaging amyloid in brain in vivo. In addition, these methods are also important for research into AD, for which there are no known preventive or therapeutic means available yet, and for diagnosis patients of Down's syndrome featured by amyloid-containing neuritic plaques and persons homozygous for the apolipoprotein E4 allele who are very likely to develop AD.
Even though various attempts have been made to diagnose AD in vivo, there are no antemortem probes for brain amyloid yet, and thus far no in vivo methods for AD diagnosis have been demonstrated to meet these criteria.
Although additional radioligand binding sites have been identified, the currently known status for radioligand binding is that there are three different radioligand binding sites developing in each β-amyloid deposits, including binding sites of Congo red, Thioflavin T and FDDNP (Cai, Innis, and Pike, Cur. Med. Chem., 2007, 14, 19-52).
Recently, laboratory results have been reported, suggesting that Congo red inhibits β-amyloid induced neurotoxicity and cell degeneration in vitro. (Burgevin et al., NeuroReport, 1994, 5, 2429; Lorenzo and Yankner, Proc. Natl. Acad. Sci., 1994, 91, 12243-12247; Pollack et al., Neuroscience Letters, 1995, 184, 113; Pollack et al., Neuroscience Letters, 1995, 197, 211). The mechanism appears to involve both inhibition of fibril formation and prevention of the neurotoxic properties of formed fibrils (Lorenzo and Yankner, Proc. Natl. Acad. Sci., 1994, 91, 12243-12247). Congo red is also known to protect pancreatic cells from toxic amylin, which is similar to β-amyloid plaques or fibrils of the pancreas of patients' with Type II diabetes (Lorenzo and Yankner, Proc. Natl. Acad. Sci., 1994, 91, 12243-12247). These findings imply that a compound that specifically binds to β-amyloid deposits and inhibits β-amyloid plaque formation can be a composition candidate of a single therapeutic agent or a combination of therapeutic agents for preventing or treating AD associated with β-amyloid deposits.
Under the circumstances, the present inventors have researched preparation methods of novel compounds for diagnosis and treatment of AD and found that when the pharmaceutical composition containing the 2-arylbenzothiophene derivative binds with a low-molecular weight β-amyloid peptide binding compound, generation of malignant high-molecular weight β-amyloid deposits is suppressed. Accordingly, the pharmaceutical composition can be used as a therapeutic agent of a degenerative brain disease such as Alzheimer's disease. Based on these findings, the inventors of the present invention have completed the invention.